Polyimides commonly are used as interlayer insulators, protective layers, or etch masks in the manufacture of electronic components because of their superior heat and chemical resistance, and dielectric properties. Photoresist compositions used to create patterned polyimide film for these applications are well known in the art. Such polyimide films typically are deposited from a solution as a coating. When exposed imagewise to light, solubility of the film is altered in the imaged areas.
The polyimide film resists generally are negative-working, but may be positive-working. Negative-working embodiments primarily are based on polyimides or polyimide precursors, such as polyamic esters containing ethylenically unsaturated double bonds (e.g., acrylic ester moieties in the side chain). The acrylic moieties crosslink during uv irradiation, causing exposed regions to become insoluble in organic solvents. Unexposed regions remain soluble in organic solvents, and thus are removed by the appropriate solvent. Large quantitites of solvent are needed for complete removal of the unexposed regions and resolution of the developed image is limited due to swelling of the crosslinked exposed regions.
Most of the positive-working polyimide resists that have been reported are based on the standard quinonediazide photochemistry and poly(amic acid) wherein the solubility of poly(amic acid), in alkaline developer solution is inhibited by the quinonediazide compounds. Irradiation renders the exposed areas soluble in the developer solution. However, it is known that such resist formulations usually have a limited storage life because the quinonediazide compounds decompose relatively rapidly in the presence of acid. Moreover, the difference in solubility between the exposed and unexposed areas is relatively small and their stability to alkaline development and etching solutions is unsatisfactory.
Incorporation of photoactive groups is another approach that has been used in positive-working polyimide resists. An example is the o-nitrobenzyl ester of poly(amic acid) in which the o-nitrobenzyl ester groups are photochemically converted to acid groups, rendering the imaged areas more soluble in an alkaline developer solution.
The prior art methods discussed above are based on photochemical processess that depend upon the use of unfriendly materials, such as monomers, and delicately balanced photosensitive formulations. Also, elaborate synthesis is required to prepare the photoactive polyimide precursors. The processes are usually complex, involving the preparation of a photomask, exposure with registraion, and development. The finished product may not have all the desired mechanical and electrical properties because compromises usually are needed to achieve the desired photochemical properties.
The present invention overcomes many of these disadvantages and provides a process which poses minimal environmental risks, is easy to use and has the capability of rapidly producing images of high resolution.